Method and apparatus for minimizing interruption time of data transfer on handover in wireless communication network

ABSTRACT

To solve the problem wherein a terminal cannot perform data communication from a network from the time point of a terminal receiving a handover command (HO command) message from a base station controlling a source cell to the time point of a terminal receiving a handover complete (HO complete) message from a base station controlling a target cell, after the terminal transmits the handover command message to the base station controlling the source cell, the base station controlling the source cell transmits to the base station controlling the target cell whether or not the terminal supports a function of reducing the interruption time of data transfer, the terminal continues to perform data transmission/reception with the base station controlling the source cell, and after the terminal completes a random access procedure with the base station controlling the target cell, the base station controlling the source cell transmits an additional sequence number status transfer message to the base station controlling the target cell.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation application of prior application Ser.No. 16/072,300, filed on Jul. 24, 2018, which is a U.S. National Stageapplication under 35 U.S.C. § 371 of an International application numberPCT/KR2017/000562, filed on Jan. 17, 2017, which was based on andclaimed priority under 35 U.S.C § 119(a) of a Korean patent applicationnumber 10-2016-0019916, filed on Feb. 19, 2016, in the KoreanIntellectual Property Office, the disclosure of which is incorporated byreference herein in its entirety.

TECHNICAL FIELD

The present invention relates to a method and apparatus for improving acell switching procedure (i.e., handover procedure) of a terminal thatcross a cell boundary in a cellular wireless communication network.

BACKGROUND ART

FIG. 1 is a diagram illustrating a handover (HO) procedure forserving-cell switching from a source cell to a target cell. Hereinafter,the source cell is interchangeably referred to as cell A 100, the targetcell is interchangeably referred to as cell B 120, a base stationhosting the cell A 100 is interchangeably referred to as enhanced node B(eNB) A 110, and a base station hosting the cell B is interchangeablyreferred to as eNB B 130.

If a terminal (interchangeable referred to as mobile terminal and userequipment (UE)) moves from one cell to another across a cell boundary,the cellular wireless communication system performs a HO procedure forswitching the serving cell from a source cell to a target cell. In anexemplary 3GPP LTE network system as depicted in FIG. 1, if a UE 140moves from the cell A 100 to the cell B 120, the eNB A 100 performs theHO of the UE 140 to the eNB B 130.

FIG. 2 is a signal flow diagram illustrating a schematic HO procedure ina long term evolution (LTE) system.

In FIG. 2, if the eNB A 110 sends the eNB B 120 a HO request message torequest at step S200, the eNB B 130 determine whether it can accept therequest and, if so, sends the eNB A 110 a HO request acknowledgement(ACK) message, at step S210, to accept the HO request. The eNB A sendsthe UE 140 a HO command message, at step S220, to instruct to switch itsserving cell to the cell B 120 and sends, at step S230, the eNB B 130 asequence number (SN) status transfer message carrying the informationabout downlink (DL)/uplink (UL) data for data forwarding to the eNB B130 at step S240. Upon receipt of the HO command message, the UE 140stops data communication with the cell A 100 and initiates a procedurefor a call connection to the cell B 120. In this procedure, the UEperforms a random access procedure by sending a random access channel(RACH) to the eNB B 120 at step S250, and the eNB B 130 sends a randomaccess response (RAR) message in reply at step S260. Finally, the UE 140sends the eNB B 120 a HO complete message to compete the HO procedurewith successful serving cell switching to the cell B 120. Afterward, theUE resumes data communication via the cell B 120.

DISCLOSURE OF INVENTION Technical Problem

In the conventional LTE HO scheme, however, the UE has to suspend datacommunication with the network until the eNB B receives the HO completemessage from the UE since the time point when the UE receives the HOcommand message from the eNB A. Such a data communication suspensionperiod is referred to as data interrupt time. In the LTE system wherethe data transfer interruption time mounts typically up to a fewmilliseconds, if the UE on which an application interacting with thenetwork in real time moves across a cell boundary (requiring ahandover), this means that the real-time interaction with the networkbecomes impossible during the data transfer interruption time. Forexample, if the UE moves across cell boundaries frequently in the courseof receiving a cloud game service, it may be difficult to expect asmooth progress of the game or any real-time call or conferencerequiring real-time video transfer.

With the trend of miniaturization and compactification of futurenetworks, the number of times of cell boundary crossings will increase,which will be likely to be one of the main causes making it difficult toguarantee real-time interaction with the network in combination with thedata transfer interruption time of the current LTE technology.

Solution to Problem

In accordance with an aspect of the present invention, a handover methodof a base station controlling a source cell includes transmitting ahandover command to a terminal, transmitting a sequence number statustransfer message and information indicating whether the terminalsupports a data transfer interruption time reduction function to a basestation controlling a target cell, and communicating a data packet withthe terminal, wherein the terminal communicates the data packet with thebase station controlling the source cell and, simultaneously, performs arandom access procedure with the base station controlling the targetcell.

In accordance with another aspect of the present invention, a handovermethod of a base station controlling a target cell includes receiving asequence number status transfer message and information indicatingwhether a terminal supports a data transfer interruption time reductionfunction from a base station controlling a source cell and performing arandom access procedure with the terminal, wherein the terminalcommunicates a data packet with the base station controlling the sourcecell and, simultaneously, perform the random access procedure with thebase station controlling the target cell.

In accordance with another aspect of the present invention, a handovermethod of a terminal being handed over from a source cell to a targetcell includes receiving a handover command from a base stationcontrolling a source cell, performing, after receiving the handovercommand, a random access procedure with a base station controlling atarget cell at the same time as communicating a data packet with thebase station controlling the source cell, and transmitting, aftercompleting the random access procedure, a handover complete message tothe base stations controlling the source and target cells.

In accordance with another aspect of the present invention, a basestation controlling a source cell for a handover includes a transceiverconfigured to communicate signals with a terminal and a base stationcontrolling a target cell and a controller configured to control totransmit a sequence number status transfer message and informationindicating whether the terminal supports data transfer interruption timereduction function to the base station controlling the target cell,communicate a data packet with the terminal, receive a handover completemessage from the terminal or a terminal(UE) context release from thebase station controlling the target cell, and transmit an additionalsequence number status transfer message to the base station controllingthe target cell after receiving the handover complete message or theterminal context release message, wherein the terminal communicates thedata packet with the base station controlling the source cell and,simultaneously, performs a random access procedure with the base stationcontrolling the target cell simultaneously, and the additional sequencenumber status transfer message includes information related to datapackets communicated with the terminal after transmitting the sequencenumber status transfer message.

In accordance with another aspect of the present invention, a basestation controlling a target cell for a handover includes a transceiverconfigured to communicate signals with a base station controlling asource cell and a terminal and a controller configured to control toreceive a sequence number status transfer message and informationindicating whether a terminal supports a data transfer interruption timereduction function from a base station controlling a source cell andperform a random access procedure with the terminal, wherein theterminal communicates a data packet with the base station controllingthe source cell and, simultaneously, performs the random accessprocedure with the base station controlling the target cell.

In accordance with still another aspect of the present invention, aterminal being handed over from a source cell to a target cell includesa transceiver configured to communicate signals with a base stationcontrolling a source cell and a base station controlling a target celland a controller configured to control to receive a handover commandfrom a base station controlling a source cell, perform, after receivingthe handover command, a random access procedure with a base stationcontrolling a target cell at the same time as communicating a datapacket with the base station controlling the source cell, transmit,after completing the random access procedure, a handover completemessage to the base stations controlling the source and target cells,receive, if the data packet is a downlink data packet, the data packetfrom the base station controlling the target cell after the base stationcontrolling the target cell receives an additional sequence numberstatus transfer message, and transmit, if the data packet is a uplinkdata packet, the data packet to the base station controlling the targetcell after transmitting the handover complete message, wherein theestimated sequence number status information is information related to asequence number of a data packet predicted to be transmitted from thebase station controlling the source cell to the terminal beforereceiving the handover complete message.

Advantageous Effects of Invention

According to an embodiment of the present invention, it may be possibleto provide a method and apparatus for minimizing the data interrupt timefor a UE to perform a HO in a cellular wireless communication network.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating a handover (HO) procedure forserving-cell switching from a source cell to a target cell;

FIG. 2 is a signal flow diagram illustrating a schematic HO procedure ina long term evolution (LTE) system;

FIG. 3 is a diagram illustrating a DL data radio bearer (DRB) operationfor a HO when a radio link control (RLC) layer operates in anacknowledged mode (AM);

FIG. 4 is a diagram illustrating a DL DRB operation for a HO when an RLClayer operates in an unacknowledged mode (UM);

FIG. 5 is a diagram illustrating a DL traffic flow in a HO procedure;

FIG. 6 is a diagram illustrating UL traffic flow in a HO procedure;

FIG. 7 is a diagram illustrating an example of a UE crossing cellboundaries;

FIGS. 8A and 8B are diagrams illustrating a HO procedure and a DL dataflow during the HO procedure according to an embodiment of the presentinvention;

FIG. 9 is a flowchart illustrating an DL traffic transmission operationof the eNB A 110 during a HO;

FIG. 10 is a flowchart illustrating a DL traffic transmission operationof the eNB B 130 during a HO;

FIG. 11 is a flowchart illustrating an operation of the UE 140supporting a data transfer interruption time reduction function;

FIG. 12 is a diagram illustrating DL data flows among eNB A, eNB B, anda UE in a HO procedure according to an embodiment of the presentinvention;

FIG. 13 is a diagram illustrating UL data flows among eNB A, eNB B, anda UE in a HO procedure according to an embodiment of the presentinvention;

FIG. 14 is a flowchart illustrating a UL traffic reception operation ofthe eNB A 110 during a HO;

FIG. 15 is a flowchart illustrating a UL traffic reception operation ofthe eNB B 130 during a HO;

FIG. 16A is a block diagram illustrating a configuration of an eNB forperforming the present invention;

FIG. 16B is a block diagram illustrating another configuration of an eNBfor performing the present invention;

FIG. 17A is a block diagram illustrating a configuration of a UEsupporting a data transfer interruption time reduction function forperforming the present invention; and

FIG. 17B is a block diagram illustrating another configuration of a UEsupporting a data transfer interruption time reduction function forperforming the present invention.

MODE FOR THE INVENTION

Exemplary embodiments of the present invention are described withreference to the accompanying drawings in detail. Detailed descriptionof well-known functions and structures incorporated herein may beomitted to avoid obscuring the subject matter of the present invention.Further, the following terms are defined in consideration of thefunctionality in the present invention, and may vary according to theintention of a user or an operator, usage, etc. Therefore, thedefinition should be made on the basis of the overall content of thepresent specification.

Although the description is directed to an orthogonal frequency divisionmultiplexing-based (OFDM-based) radio communication system, particularlya 3^(rd) generation partnership project (3GPP) LTE and LTE-Advanced(LTE-A) standard, it will be understood by those skilled in the art thatthe present invention can be applied even to other communication systemshaving the similar technical background and channel format, with aslight modification, without departing from the spirit and scope of thepresent invention.

Advantages and features of the present invention and methods ofaccomplishing the same may be understood more readily by reference tothe following detailed description of exemplary embodiments and theaccompanying drawings. The present invention may, however, be embodiedin many different forms and should not be construed as being limited tothe exemplary embodiments set forth herein. Rather, these exemplaryembodiments are provided so that this invention will be thorough andcomplete and will fully convey the concept of the invention to thoseskilled in the art, and the present invention will only be defined bythe appended claims. Like reference numerals refer to like elementsthroughout the specification.

FIG. 3 is a diagram illustrating a DL data radio bearer (DRB) operationfor a HO when a radio link control (RLC) layer operates in anacknowledged mode (AM).

In reference to FIG. 3, data forwarding is performed in a packet dataconvergence protocol (PDCP) layer during a HO procedure, and a PDCPlayer 310 of the eNB A forwards a PDCP service data unit (SDU)(hereinafter, interchangeably referred to as data packet) that has beenreceived from a serving gateway (S-GW) 300 and then transmitted to theUE but no ACK corresponding thereto has been received, i.e., PDCP SDU 7in FIG. 3, and a PDCP SDU of which transmission is not attempted yet,i.e., PDCP SDU 9 in FIG. 3, to a PDCP layer 320 of the eNB B. The eNB Btransmits all PDCP SDUs received through data forwarding to the PDCPlayer 330 of the UE.

FIG. 4 is a diagram illustrating a DL DRB operation for a HO when an RLClayer operates in an unacknowledged mode (UM).

In reference to FIG. 4, a PDCP layer 410 of the eNB A forwards only thePDCP PDU of which transmission is not attempted yet, i.e., PDCP PDU 9 inFIG. 4, to a PDCP layer 420 of the eNB B regardless of thesuccess/failure of previous data transmission unlike the case of FIG. 3.The eNB B transmits all of the PDCP SDUs received through the dataforwarding to the PDCP layer 420 of the UE after the completion of theHO procedure.

FIG. 5 is a diagram illustrating a DL traffic flow in a HO procedure.

In reference to FIG. 5, the UE 140 receives data from the eNB A 110until the HO command message is received from the eNB A 110 and, at thistime, the data flows from the S-GW to the UE 140 through the PDCP andRLC layers of the eNB A 110. This traffic flow is denoted by referencenumber 5(1) in FIG. 5.

The UE 140 cannot receive the data from the network during a periodbetween a time point when the UE 140 receives a HO command message and atime point when the eNB B 130 receives a HO complete message from the UE140, and the data transmitted from the S-GW to the PDCP layer of the eNBA 110 during this period are forwarded to the PDCP layer of the eNB B130. This traffic flow is denoted by reference number 5(2) in FIG. 5.

Since the time point when the eNB B 130 receives the HO complete messagefrom the UE 140, the data flows from the S-GW to the PDCP layer of theeNB B and then to the UE 140 through the RLC layer of the eNB B 130.

FIG. 6 is a diagram illustrating UL traffic flow in a HO procedure.

In accordance with FIG. 6, the UE 130 transmits data to the eNB A 110until it receives a HO command message from the eNB A 110, and the ULdata flow to the S-GW through the RLC and PDCP layers of the eNB A 110.This traffic flow is denoted by reference number 6(1) of FIG. 6.

The UE 140 cannot transmit data to the network during a period between atime point when it receives a HO command message and a time point whenthe eNB B 130 receives a HO complete message from the UE 140. Thistraffic flow is denoted by reference number 6(2) in FIG. 6.

Since the time point when the eNB B 130 receives the HO complete messagefrom the UE 140, the UE 140 transmits the UL data via the eNB 130, andthe data flows to the S-GW through the RLC and PDCP layers of the eNB B.This traffic flow is denoted by reference number 6(3) in FIG. 6.

According to the legacy LTE HO scheme designed to operate as describedwith reference to FIGS. 5 and 6, the UE cannot perform datacommunication with the network until the eNB B receives the HO completemessage since the UE has received the HO command message from the eNB A.Particularly when the UE crosses cell boundaries frequently as shown inFIG. 7, the problem becomes worse. Therefore, the present invention aimsto provide a method and apparatus for minimizing the data transferinterruption time during a UE handover in a cellular wirelesscommunication network.

FIGS. 8A and 8B are diagrams illustrating a HO procedure and a DL dataflow during the HO procedure according to an embodiment of the presentinvention.

In the embodiment of FIG. 8A, a UE 140 continues data communication withits old serving cell (in this case, cell A 100), even after receiving aHO command message from eNB A 110, and simultaneously performs a callconnection procedure to connect to cell B 120. That is, the UE 140 mayattempt a random access to the cell B 120, receive an RAR in reply,exchange additionally necessary information, and finally send a HOcomplete message to the eNB A 110 and eNB B 130, in the course of thedata communication with the cell A 100. In the embodiment of FIG. 8B,the UE 140 may send the HO complete message only to the eNB B 130, andthen the eNB B 130 may send the eNB A 110 a UE context release messageto the eNB A 110 at step S840.

Then, the UE starts data communication with the cell B 120, and the eNBA 110 sends a sequence number (SN) status transfer message for the dataaddressed to the UE 140 to the eNB B 130; this scheme may reduce thedata transfer interruption time during the HO for the UE dramatically.However, such a procedure is applicable to only the UEs that supports adata transfer interruption time reduction function, the legacy HO schemeis applied to the UEs that do not support the data transfer interruptiontime reduction function.

According to an embodiment of the present invention, a UE transmitsignals to two eNBs simultaneously as described above and, in order toaccomplish this, it has to activate two radio frequency (RF) units andsignal processing units. This is similar to the case of activating twoor more RF units and signal processing units for a primary cell (PCell)and secondary cell (SCell) in a carrier aggregation (CA) mode butdiffers in terms of processing signals on the same frequency from the CAmode.

According to an embodiment of the present invention, the UE 140determines whether it supports the data transfer interruption timereduction function and whether the information on estimated SN status(N) is transmitted from the eNB A 110 to the eNB B 130 in the course ofdata forwarding at step S810 or SN status transfer message transmissionat step S800 abiding by the HO procedure of the legacy standard. Theestimated NS status (N) indicates the NS status of a PDCP SDU that isestimated to be transmitted from the eNB A 110 to the UE 140 until thetime point of receiving the HO complete message, and it is determinedbased on a sequence number of the earliest PDCP SDU (N_(o)) remaining inthe eNB A 110, the average data rate (R_(A)) (downlink data rate to theUE) of the eNB A 110, and latency (T_(HO)(eNB_(A),eNB_(A))) from thedata forwarding to the HO complete message reception during the HO fromthe eNB A 110 to the eNB B 130 as expressed by [Equation 1].N=f(N _(O) ,R _(A) ,T _(HO)(eNB _(A) ,eNB _(A)))  [Equation 1]

Unlike the legacy HO procedure in which the PDCP SDUs remaining in theeNB A after the data forwarding are discard, the PDCP SDUs, in anembodiment of the present invention, are transmitted, rather than beingdiscarded, to the UE until the data communication between the eNB A andthe UE is broken or the eNB A receives the HO complete message from theUE at step S820 (or UE context release message from the eNB B that hasreceived the HO complete message from the UE). Then, the eNB A 110 sendsthe eNB B 130 an additional SN status transfer message at step S830. Theadditional SN status transfer message includes the information on the SNof the PDCP SDU that is indicated as a missing PDCP PDU for the UE and,afterward, determined as a PDCP PDU received by the UE. In the AM mode,the eNB A 110 makes a notification to the eNB B 130 reflecting all ofpositive/negative acknowledgements (ACK/NACK); in the UM mode, the eNB A110 makes a notification to the eNB B 130 of the last PDCP SDU it hastransmitted, immediately upon receipt of the HO complete message fromthe UE 140.

The eNB B 130 starts transmitting data to the UE 140 after receiving theHO complete message or transmitting the UE context release message, fromthe PDCP SDU with the SN (N+δ) at the eNB A 110 rather than the PDCP SDUwith the earliest SN at the eNB B. Here, δ is a predetermined integerconstant. That is, δ may be set to 0 and a negative number. Afterreceiving the additional SN status transfer message from the eNB A 110,the eNB B 130 discards the PDCP SDUs that have been already transmittedfrom the eNB A 110 to the UE 140 based on the additional SN statusinformation and starts transmission from the PDCP SDU with the earliestSN remaining after the discard. It may also be possible for the eNB B130 to wait for the additional SN status transfer message from the eNB A110, after the receipt of the HO complete message, and starttransmission to the UE 140 from the PDCP SDU with the earliest SN byreflecting the information carried by the additional SN status transfermessage.

FIG. 9 is a flowchart illustrating a DL traffic transmission operationof the eNB A 110 during a HO.

In reference to FIG. 9, the eNB A 110 sends n HO command to the UE 140at step 900. Next, the eNB A determines at step 910 whether thecorresponding UE supports a data transfer interruption time reductionfunction. If the UE supports the data transfer interruption timereduction function, the eNB A calculates an estimated NS status at step920 as described above and, at step 930, sends the eNB B 130 the SNstatus, data, estimated SN status, and information indicating whetherthe corresponding UE supports the data transfer interruption timereduction function.

Next, the eNB A performs data communication with the UE at step 950,determines at step S960 whether a HO complete message is received,sends, if the HO complete message is received, the eNB B an additionalSN status transfer message at step 970, and continues, if the HOcomplete message is not received, the data communication with the UE atstep 950.

If it is determined that the UE does not support the data transferinterruption time reduction function, a legacy HO scheme is applied tothe HO for the UE between the eNB A and the eNB B at step 940.

The UE may send the eNB A the UE capability including informationindicating whether it supports the data transfer interruption timereduction function when it accesses the eNB A, or the eNB A may receivethe information indicating whether the UE supports the data transferinterruption time reduction function from a mobility management entity(MME).

FIG. 10 is a flowchart illustrating a DL traffic transmission operationof the eNB B 130 during a HO.

In reference to FIG. 10, the eNB B 130 receives SN status, data,estimated SN status, and information indicating whether the UE supportsthe data transfer interruption time reduction function from the eNB A110 and determines at step 1000 whether the UE supports the datatransfer interruption time reduction function. If it is determined thatthe corresponding UE supports the data transfer interruption timereduction function, the eNB B 130 starts, at step 1010, transmission tothe UE from the PDCP SDU with the SN of N+δ at the eNB A 110 afterreceiving a HO complete message from the UE 140. As described above,this step may be omitted. Next, the eNB B 130 receives an additional SNstatus transfer message from the eNB A at step 1020 and discards, atstep 1040, the stored PDCP SDUs based on the received additional SNstatus. Then, the eNB B 130 sends data to the UE 140 from the PDCP SDUwith the earliest SN at step 1050.

If it is determined that the UE does not support the data transferinterruption time reduction function, a legacy HO scheme is applied tothe HO for the UE between the eNB A and the eNB B at step 1030.

FIG. 11 is a flowchart illustrating an operation of the UE 140supporting a data transfer interruption time reduction function.

In reference to FIG. 11, the UE 140 receives a HO command message atstep 1100. Next, the UE activates two RF units and two signal processingunits at step 1110. The two RF units are a first RF unit and a second RFunit, and the two signal processing units are a first signal processingunit and a second processing unit, the first RF unit and signalprocessing unit being responsible for signal communication and signalprocessing with the eNB A 110 and the second RF unit and signalprocessing unit being responsible for signal communication and signalprocessing with the eNB B 130. That is, the first RF unit and the firstsignal processing unit are used for data communication with the eNB A110, and the second RF unit and the second signal processing unit areused for a random access to the eNB B 130 at step 1120.

The UE determines at step 1130 whether the random access procedure withthe eNB B is completed and, if the random access procedure is completedbetween the UE and the eNB B, releases the connection with the eNB A anddeactivates the first RF unit and the first signal processing unit andperforms data communication with the eNB B by means of the second RFunit and the second signal processing unit at step 114. If the randomaccess procedure is not completed between the UE and the eNB B, the UEcontinues the data communication with the eNB A and the random accessprocedure with the eNB B at step 1120.

FIG. 12 is a diagram illustrating DL data flows among eNB A, eNB B, anda UE in a HO procedure according to an embodiment of the presentinvention.

In reference to FIG. 12, the data flow during a period from the HOcommand message transmission/reception time point to the HO completemessage transmission time point as denoted by reference number 12(2) inFIG. 12 differs from that as denoted by reference number 5(2) in FIG. 5depicting the legacy HO scheme. In the embodiment of FIG. 12, the dataflows from the PDCP layer of the eNB A 110 simultaneously to the PDCPlayer of the eNB B 110 and the RLC layer of the eNB A 110 and then tothe UE 140 as denoted by reference number 12(2) unlike the case asdenoted by reference number 5(2) in FIG. 5 where the data received fromthe S-GW flows from the PDCP layer of the eNB A to the PDCP layer of theeNB B.

In comparison with FIG. 5, the eNB A 110 sends the eNB B 130 theinformation indicating whether the UE supports the data transferinterruption time reduction function along with the SN status transfermessage at step S1200, the UE 140 sends the HO complete message to theeNB A 110 as well as the eNB B 130 at step S1210, and the eNB A 110sends the additional SN status transfer message to the eNB B 130 at stepS1220 in the embodiment of FIG. 12.

FIG. 13 is a diagram illustrating UL data flows among eNB A, eNB B, anda UE in a HO procedure according to an embodiment of the presentinvention.

In reference to FIG. 13, the data flow during a period from the HOcommand message transmission/reception time point to the HO completemessage transmission time point as denoted by reference number 12(2) inFIG. 12 differs from that as denoted by reference number 6(2) in FIG. 6depicting the legacy HO scheme. In the embodiment of FIG. 13, the datatransmitted by the UE flows to the S-GW through the RLC layer of the eNBA as denoted by reference number 13(2) unlike the case as denoted byreference number 6(2) in FIG. 6 where there is no data flow among theUE, the eNB A, and eNB B.

That is, the corresponding UE may perform a call connection procedure toaccess the cell B 120 in the course of the data communication with thecell A 100. The UE may attempt a random access in the cell B 120,receive an RAR in reply, exchange additionally necessary information,and finally send a HO complete message to the eNB B 130. Afterward, theUE resumes data communication with the cell B 120, and the eNB A 100sends the eNB B 120 the information on the receipt of a PDCP protocoldata unit (PDU) (which is interchangeably referred to as data packet)from the UE during a period from the HO command message transmissiontime point or SN status transfer message transmission time point to theHo complete message reception time point; this scheme may reduce thedata transfer interruption time during the HO for the UE dramatically.

In comparison with FIG. 6, the eNB A 110 sends the eNB B 130 theinformation indicating whether the UE supports the data transferinterruption time reduction function along with the SN status transfermessage at step S1300, the UE 140 sends the HO complete message to theeNB A 110 as well as the eNB B 130 at step S1310, and the eNB A 110sends the eNB B 130 the additional SN status transfer message at stepS1320 in FIG. 13.

FIG. 14 is a flowchart illustrating a UL traffic reception operation ofthe eNB A 110 during a HO.

In reference to FIG. 14, the eNB A 110 sends the UE 140 a HO commandmessage at step 1400. Next, the eNB A determines whether thecorresponding UE supports the data transfer interruption time reductionfunction at step 1410 and, if so, sends the eNB B 130 the informationindicating that the UE supports the data transfer interruption timereduction function along with SN status at step 1420. Afterward, the eNBA performs data communication with the UE, i.e., receives UL data fromthe UE, at step 1430, and determine at step 1440 whether the datacommunication with the UE is broken or a HO complete message isreceived. If the data communication with the UE is broken or a HOcomplete message is received, the eNB sends the eNB B 130 an additionalSN status transfer message at step 1460. If neither the datacommunication with the UE is broken nor a HO complete message isreceived, the eNB A continue the data communication with the UE at step1430.

If the corresponding UE does not support the data transfer interruptiontime reduction function, a legacy HO scheme is applied to the HO for theUE between the eNB A and the eNB B at step 1440.

In comparison with FIG. 9 depicting the case where the eNB A relays DLtraffic, the eNB A neither calculates any estimated SN status norperform data forwarding. This is because the UL traffic carries datafrom the UE.

FIG. 15 is a flowchart illustrating a UL traffic reception operation ofthe eNB B 130 during a HO.

In reference to FIG. 15, the eNB B 130 determines at step 1500 whether aUE supports a data transfer interruption time reduction function basedon the SN status and information indicating whether the corresponding UE140 supports the data transfer interruption time reduction function thatare received from the eNB A 110. If it is determined that thecorresponding UE supports the data transfer interruption time reductionfunction, the eNB B 130 receives data from the UE after receiving a HOcomplete message from the UE 140, i.e., performs data communication withthe UE, at step 1510. Next, the eNB B 130 determines at step 1520whether an additional SN status transfer message is received from theeNB A and, if the additional SN status transfer message is received,reflects the received SN status at step 1540 and continues datacommunication with the UE at step 1550. If the additional SN statustransfer message is not received, the eNB B continues data communicationwith the UE at step 1510.

If the corresponding UE does not support the data transfer interruptiontime reduction function, a legacy HO scheme is applied to the HO for theUE between the eNB A and the eNB B at step 1530.

Here, the operation of the UE that has been already described withreference to FIG. 11 is omitted. However, the UE 140 may perform PDCPPDU transmission subsequently to the transmission to the eNB A at thetime point of releasing the connection with the eNB A 110 and startingdata communication with the eNB B 130 and retransmit corresponding PDUsfrom the beginning.

FIG. 16A is a block diagram illustrating a configuration of an eNB forperforming the present invention.

In reference to FIG. 16A, the eNB (base station) 1600 includes a UEfunction management unit 1610, a data storage unit 1620, an inter-eNBcommunication unit 1630, a data communication control unit 1625, asignal processing unit 1635, an RF unit 1640, and a controller 1605.

The individual blocks operate as follows. The UE function managementunit 1620 manages whether the UEs being served by the eNB support thedata transfer interruption time reduction function and notify thecontroller 1605 whether a UE for which a handover is required supportsthe data transfer interruption time reduction function. The data storageunit 1620 may store data to be delivered to all UEs and send the data tothe data communication control unit 1625 and the inter-eNB communicationunit 1630 under the instruction of the controller 1605.

The inter-eNB communication unit 1630 is responsible for communicationwith other eNBs and, if there are DL data remaining in the source cellduring a HO, forwards the DL data to (an eNB controlling) the targetcell such that the UE receives the DL data from the target cell. Thedata communication control unit 1625 controls to use constrainedresources efficiently for transmitting data fetched from the datastorage unit 1620 to the UE.

The signal processing unit 1635 generates an actual baseband signal inaccordance with a resource allocation result from the data communicationcontrol unit 1625 in transmitting a DL signal and processes a signalreceived by the RF unit 1640 in receiving a UL signal. The RF unit 1640transmits the baseband signal generated by the signal processing unit tothe UE in a radio frequency band (radio band) in transmitting the DLsignal and receives the signal transmitted by the UE in the radiofrequency band in receiving the UL signal.

The controller 1605 controls functions for data communication with theUE and HO for the UE entirely. During a handover for an arbitrary UE,the controller 1605 sends the UE function management unit 1610 a requestfor indication on a data transfer interruption time reduction functionsupportability to receive the information on the data transferinterruption time reduction function supportability and, if the UE doesnot support the data transfer interruption time reduction function,stops data transfer from the data storage unit 1620 to the datacommunication control unit 1625 and control the inter-eNB communicationunit 1630 to transmit an SN status and all of the data stored in thedata storage unit 1620 to the target cell. If the UE supports the datatransfer interruption time reduction function, the controller 1605calculates an estimated SN status (N) and controls to continue datatransfer from the data storage unit 1620 to the data communicationcontrol unit 1625 and to send the SN status, Estimated SN status, alldata stored in the data storage unit 1620, and the additional SN statusto the target cell by means of the inter-eNB communication unit 1630.

According to the 3GPP LTE standard, the data storage unit 1620 may beincluded in the RLC layer, the data communication control unit 1625 maybe included in a media access control (MAC) layer, and the signalprocessing unit 1635 may be included in a physical layer. The inter-eNBcommunication unit 1630 may be a block that is responsible for X2interface-based inter-eNB communication.

FIG. 16B is a block diagram illustrating another configuration of an eNBfor performing the present invention.

In reference to FIG. 16B, the eNB 1600 may include a controller 1650 anda transceiver 1660, and the controller 1650 may be responsible for thefunctions of the UE function management unit 1610, the data storage unit1620, the inter-eNB communication unit 1630, the data communicationcontrol unit 1625, and the controller 1605 of FIG. 16A. The transceiver1660 may be responsible for the functions of the signal processing unit1635 and the RF unit 1640.

FIG. 17A is a block diagram illustrating a configuration of a UEsupporting a data transfer interruption time reduction function forperforming the present invention.

In reference to FIG. 17A, the UE 1710 includes a function managementunit 1710, a first RF unit 1730, a second RF unit 1735, a first signalprocessing unit 1720, a second signal processing unit 1725, a data andcall connection communication unit 1715, and a controller 1705.

The individual blocks operate as follows. The function management unit1710 verifies that the corresponding UE supports the data transferinterruption time reduction function and notifies the controller 1705that the UE supports the data transfer interruption time reductionfunction, which is notified to an eNB. The RF units 1730 and 1735receive signals transmitted by the eNB through a radio frequency band inreceiving DL signals and send the eNB the baseband signals generated bythe first and second signal processing units 1720 and 1725 through theradio frequency band in transmitting UL signals. During a handover, thefirst RF unit 1730 of the two RF units is responsible for the signal ofa source cell, and the second RF unit 1735 is responsible for a physicalsignal of a target cell. The signal processing units 1720 and 1725process the signals received by the RF units in receiving DL signals andgenerate actual baseband signals in transmitting UL signals. During ahandover, the first signal processing unit of the two signal processingunits is responsible for the physical signals of the source cell, andthe second signal processing unit is responsible for the physicalsignals of the target cell.

The data and call connection communication unit 1715 is responsible fordata communication and call connection communication with the eNB. Thecontroller 1705 controls the signal processing units 1720 and 1725 tooperate for the purposes such as call connection and data communication.That is, the call connection is controlled by means of the second signalprocessing unit 1725 so as to use the signal of the first signalprocessing unit 1720 before transmitting a HO complete message and thenthe signal of the second signal processing unit 1725 after transmittingthe HO complete message. Also, the UL transmission signal is processedby the first signal processing unit 1720 before the HO complete messagetransmission and then the second signal processing unit 1725 after theHO complete message transmission.

The controller 1705 controls functions for data communication with theeNB and HO entirely. When a HO procedure is initiated, the controlleractivates the signal processing units 1720 and 1725 for additional useto process the signals of the source and source cells, respectively. Thecontroller is responsible for controlling the data and call connectioncommunication unit 1715 in accordance with the roles of the signalprocessing units 1720 and 1725.

FIG. 17B is a block diagram illustrating another configuration of a UEsupporting a data transfer interruption time reduction function forperforming the present invention.

In reference to FIG. 17B, the UE 1700 includes a controller 1750 and atransceiver 1760. The controller 1750 may be responsible for thefunctions of the function management unit 1710, the data and callconnection communication unit 1715, and the controller 1705. Thetransceiver 1760 may be responsible for the functions of the first RFunit 1730, the second RF unit 1735, the first signal processing unit1720, and the second signal processing unit 1725.

The present invention is capable of minimizing the data transferinterruption time during a HO procedure for switching the cell serving aUE in a cellular radio communication network. In detail, the presentinvention is capable of improving quality of a service being provided toa mobile UE by minimizing the data transfer interruption time.

The invention claimed is:
 1. A method performed by a terminal in acommunication system, the method comprising: receiving, from a sourcebase station, a handover command message indicating a handover from thesource base station to a target base station; performing a random accessprocedure with the target base station while communicating data with thesource base station based on a capability of the terminal; andtransmitting a handover complete message to the target base station,wherein a first status transfer message and data to be forwarded fromthe source base station to the target base station are transmitted fromthe source base station to the target base station after receiving thehandover command message and before transmitting the handover completemessage, and wherein a second status transfer message is transmittedfrom the source base station to the target base station aftertransmission of the handover complete message, in response to a messagetransmitted from the target base station to the source base station,wherein the second status transfer message is used to identify theearliest downlink packet data convergence protocol (PDCP) service dataunit (SDU) to be transmitted to the terminal by the target base station,and wherein the capability indicates the terminal supports a handoverprocedure that maintains a connection with the source base station afterreception of the handover command message and until releasing the sourcebase station.
 2. The method of claim 1, further comprising: receiving,from the source base station, downlink data after receiving the handovercommand message until the connection with the source base station isreleased; and receiving, from the target base station, downlink dataafter transmitting the handover complete message.
 3. The method of claim1, further comprising: transmitting, to the source base station, uplinkdata after receiving the handover command message until the handovercomplete message is transmitted; and transmitting, to the target basestation, uplink data after transmitting the handover complete message.4. The method of claim 1, wherein the first status transfer message andthe second status transfer message are associated with a sequence numberof packet data convergence protocol (PDCP).
 5. A method performed by asource base station in a communication system, the method comprising:transmitting, to a terminal, a handover command message indicating ahandover from the source base station to a target base station;transmitting, to the target base station, a first status transfermessage and data to be forwarded to the target base station aftertransmitting the handover command message and before a handover completemessage is transmitted from the terminal to the target base station,while communicating data with the terminal based on a capability of theterminal; and transmitting, to the target base station, a second statustransfer message in response to a message received from the target basestation, after the handover complete message is transmitted from theterminal to the target base station, wherein the second status transfermessage is used to identify the earliest downlink packet dataconvergence protocol (PDCP) service data unit (SDU) to be transmitted tothe terminal by the target base station, and wherein the capabilityindicates the terminal supports a handover procedure that maintains aconnection with the source base station after reception of the handovercommand message and until releasing the source base station.
 6. Themethod of claim 5, further comprising: transmitting, to the terminal,downlink data after receiving the handover command message until theconnection with the source base station is released.
 7. The method ofclaim 5, further comprising: receiving, from the terminal, uplink datauplink data after transmitting the handover command message until thehandover complete message is transmitted from the terminal to the targetbase station.
 8. A method performed by a target base station in acommunication system, the method comprising: receiving, from a sourcebase station, a first status transfer message and data to be forwardedfrom the source base station after a handover command message istransmitted from the source base station to a terminal and beforereceiving a handover complete message from the terminal; receiving, thehandover complete message from the terminal; and receiving, from thesource base station, based on a capability of the terminal, a secondstatus transfer message in response to a message transmitted to thesource base station after receiving the handover complete message,wherein the second status transfer message is used to identify theearliest downlink packet data convergence protocol (PDCP) service dataunit (SDU) to be transmitted to the terminal by the target base station,and wherein the capability indicates the terminal supports a handoverprocedure that maintains a connection with the source base station afterreception of the handover command message and until releasing the sourcebase station.
 9. The method of claim 8, further comprising:transmitting, to the terminal, downlink data after receiving thehandover complete message.
 10. The method of claim 8, furthercomprising: receiving, from the terminal, uplink data after receivingthe handover complete message.
 11. A terminal in a communication system,the terminal comprising: a transceiver; and a controller configured to:receive, from a source base station, a handover command messageindicating a handover from the source base station to a target basestation, perform a random access procedure with the target base stationwhile communicating data with the source base station based on acapability of the terminal, and transmit a handover complete message tothe target base station, wherein a first status transfer message anddata to be forwarded from the source base station to the target basestation are transmitted from the source base station to the target basestation after receiving the handover command message and beforetransmitting the handover complete message, and wherein a second statustransfer message is transmitted from the source base station to thetarget base station after transmission of the handover complete message,in response to a message transmitted from the target base station to thesource base station, wherein the second status transfer message is usedto identify the earliest downlink packet data convergence protocol(PDCP) service data unit (SDU) to be transmitted to the terminal by thetarget base station, and wherein the capability indicates the terminalsupports a handover procedure that maintains a connection with thesource base station after reception of the handover command message anduntil releasing the source base station.
 12. The terminal of claim 11,wherein the controller is further configured to: receive, from thesource base station, downlink data after receiving the handover commandmessage until the connection with the source base station is released,and receive, from the target base station, downlink data aftertransmitting the handover complete message.
 13. The terminal of claim11, wherein the controller is further configured to: transmit, to thesource base station, uplink data after receiving the handover commandmessage until the handover complete message is transmitted, andtransmit, to the target base station, uplink data after transmitting thehandover complete message.
 14. The terminal of claim 11, wherein thefirst status transfer message and the second status transfer message areassociated with a sequence number of packet data convergence protocol(PDCP).
 15. A source base station in a communication system, the sourcebase station comprising: a transceiver; and a controller configured to:transmit, to a terminal, a handover command message indicating ahandover from the source base station to a target base station,transmit, to the target base station, a first status transfer messageand data to be forwarded to the target base station after transmittingthe handover command message and before a handover complete message istransmitted from the terminal to the target base station, whilecommunicating data with the terminal based on a capability of theterminal, and transmit, to the target base station, a second statustransfer message in response to a message received from the target basestation, after the handover complete message is transmitted from theterminal to the target base station, wherein the second status transfermessage is used to identify the earliest downlink packet dataconvergence protocol (PDCP) service data unit (SDU) to be transmitted tothe terminal by the target base station, and wherein the capabilityindicates the terminal supports a handover procedure that maintains aconnection with the source base station after reception of the handovercommand message and until releasing the source base station.
 16. Thesource base station of claim 15, wherein the controller is furtherconfigured to transmit, to the terminal, downlink data after receivingthe handover command message until the connection with the source basestation is released.
 17. The source base station of claim 15, whereinthe controller is further configured to receive, from the terminal,uplink data uplink data after transmitting the handover command messageuntil the handover complete message is transmitted from the terminal tothe target base station.
 18. A target base station in a communicationsystem, the target base station comprising: a transceiver; and acontroller configured to: receive, from a source base station, a firststatus transfer message and data to be forwarded from the source basestation after a handover command message is transmitted from the sourcebase station to a terminal and before receiving a handover completemessage from the terminal, receive, the handover complete message fromthe terminal, and receive, from the source base station, based on acapability of the terminal, a second status transfer message in responseto a message transmitted to the source base station after receiving thehandover complete message, wherein the second status transfer message isused to identify the earliest downlink packet data convergence protocol(PDCP) service data unit (SDU) to be transmitted to the terminal by thetarget base station, and wherein the capability indicates the terminalsupports a handover procedure that maintains a connection with thesource base station after reception of the handover command message anduntil releasing the source base station.
 19. The target base station ofclaim 18, wherein the controller further configured to transmit, to theterminal, downlink data after receiving the handover complete message.20. The target base station of claim 18, wherein the controller furtherconfigured to receive, from the terminal, uplink data after receivingthe handover complete message.